At times, airplanes experience problems as they travel along the runway prior to takeoff. Such problems may include engine failure, tire blowout or any unexpected situation or occurrence that could affect the ability of the aircraft to become airborne and fly safely. When this occurs before the aircraft reaches the speed at which it can no longer stop before reaching the end of the runway, the takeoff is aborted and all available means are employed to bring the aircraft to a halt. If the problem occurs at this critical speed or at greater speeds, the takeoff is normally continued unless the problem is so severe that the aircraft would be incapable of flight.
If the decision to reject the takeoff is made too late or at speeds that are too high, the normal braking system of the aircraft will be unable to stop it before reaching the end of the runway. When an aircraft leaves the runway at high speeds there is a good chance that it will be damaged as a result of bouncing over rough terrain or impacting an obstacle. If the damage is severe, such as the rupturing of fuel tanks or the collapse of sections of fuselage, the results can be cataclysmic, destroying the aircraft and injuring or killing the passengers and crew. If the aircraft enters a body of water after leaving the runway, damage is still likely depending on the depth of the water and the type of bed, with the threat of drowning always being present. These dangers are of course also present when an aircraft overruns the runway during landing.
Experience has shown that the braking system of an aircraft involved in a rejected takeoff, or the braking system of an aircraft involved in an overrun landing where the reverse thruster is not functioning, cannot be relied on to stop the aircraft within a safe distance after the aircraft has left the runway and is traveling over a hard or icy surface. An aircraft is more likely to be safely stopped if the ground is soft due to the drag created by the frame and landing gear plowing through the ground. This has led to the suggestion of a soft-ground arrestor system. A problem with this concept, however, is that the material beyond the runway is usually soil, which can be quite unpredictable in its arresting capability as moisture and temperature conditions change. For example, dry clay can be hard and difficult to penetrate, making it likely that aircraft will simply ride over the surface without adequate slowing. Wet clay, on the other hand, can cause an aircraft to become quickly mired. This can cause collapse of the landing gear and can increase the chances of personal injury and damage to the aircraft. In cold climates, the moisture in clay can freeze, making the clay useless as an arrestor. At temperatures above freezing clay dries out quickly, requiring water to be added and worked into the material to maintain the desired strength.
Sand should be relatively dry to be an effective arrestor and would require a system for protecting it from rain. This could be done by providing the sand in waterproof bags, but these could be easily ruptured and the freezing of rain or snow filling the gaps between bags would make the sand less effective than required.
Shallow water ponds have also been suggested as being possible arrestors. They would, however, be ineffective in cold climates where they might freeze, and they would hinder the access of emergency vehicles and hamper efforts to evacuate passengers. Further, ponds create stagnation problems and attract wildlife that could interfere with aircraft operations.
The use of gravel beds has also been considered inasmuch as gravel does not suffer to the same degree as clay or sand from weather changes A major problem with this material, however, is the danger to aircraft engines from gravel sprayed up by the landing gear. Also, planing of the nose gear and the main landing gear wheels tends to occur at higher speeds, preventing the landing gear from sufficiently penetrating the bed.
A successful arrestor bed should not itself be the cause of landing gear collapse. By maintaining the landing gear intact the chances of structural damage to the aircraft are lessened, thereby reducing the possibility of fuel tank rupture. The arrestor bed should be capable of being quickly repaired after an overrun incident so as to be rendered operational as soon as possible. It should be capable of functioning properly in all weather extremes and should not require excessive maintenance.
As an alternative to the above materials, it has also been proposed that a foam arrestor bed be applied in aircraft safety overrun areas. While the use of foam appeared to have certain advantages over soil and water, no practical way of implementing the idea has heretofore been available.
It is an object of this invention to provide a layer of foam material in an aircraft safety overrun area which efficiently and safely slows an aircraft while minimizing the chance of personal injury or damage to the aircraft. Another object is to provide an aircraft arresting layer of foam material which is economical to produce and easy to set in place and maintain.